[PATCH net 8/9] net/mlx5e: Do not update BQL of old txqs during channel reconfiguration

[Tariq Toukan <tariqt@nvidia.com>]

During channel reconfiguration (e.g., ethtool private flags changes),
the driver can trigger a kernel BUG_ON in dql_completed() with the error
"kernel BUG at lib/dynamic_queue_limits.c:99".

The issue occurs in the following sequence:

During mlx5e_safe_switch_params(), old channels are deactivated via
mlx5e_deactivate_txqsq(). New channels are created and activated, taking
ownership of the netdev_queues and their BQL state.

When old channels are closed via mlx5e_close_txqsq(), there may be
pending TX descriptors (sq->cc != sq->pc) that were in-flight during the
deactivation.

mlx5e_free_txqsq_descs() frees these pending descriptors and attempts to
complete them via netdev_tx_completed_queue().

However, the BQL state (dql->num_queued and dql->num_completed) have
been reset in mlx5e_activate_txqsq and belong to the new queue owner,
leading to dql->num_queued - dql->num_completed < nbytes.

This triggers BUG_ON(count > num_queued - num_completed) in
dql_completed().

Fixes: 3b88a535a8e1 ("net/mlx5e: Defer channels closure to reduce interface down time")
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: William Tu <witu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/en_tx.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
index 14884b9ea7f3..a01ee656a1e7 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/en_tx.c
@@ -939,7 +939,11 @@ void mlx5e_free_txqsq_descs(struct mlx5e_txqsq *sq)
 	sq->dma_fifo_cc = dma_fifo_cc;
 	sq->cc = sqcc;
 
-	netdev_tx_completed_queue(sq->txq, npkts, nbytes);
+	/* Do not update BQL for TXQs that got replaced by new active ones, as
+	 * netdev_tx_reset_queue() is called for them in mlx5e_activate_txqsq().
+	 */
+	if (sq == sq->priv->txq2sq[sq->txq_ix])
+		netdev_tx_completed_queue(sq->txq, npkts, nbytes);
 }
 
 #ifdef CONFIG_MLX5_CORE_IPOIB
-- 
2.31.1